1. Field of the Invention
The present invention relates to a circuit for the detection of an asymmetry in the magnetization current caused by a reference alternating voltage i a magnetic modulator, in particular a magnetic modulator for zero flux detection with two virtually identical, wound magnet cores, each at least provided wit a primary winding, to be fed with a modulating current and a series circuit of an auxiliary winding and an impedance across which a voltage proportional to the magnetization current is generated during operation, it being necessary to supply the reference alternating voltage to the respective series circuits such that no resultant induction current is induced in the modulating current, which circuit contains a peak value detector coupled to the impedance of a series circuit, the strength of the output signal of said detector representing the degree of asymmetry of the magnetization current.
2. Related Art
A circuit of this type is known from the article "Zero-Flux Current Transformer For Wide-Band Precision Measurement In AC and DC HV Systems", by J. Lisser and A.J. van de Water, Proceedings of IEE Fourth International Conference on AC and DC HV Power Transmission, London 23-26 Sept. 1985, p. 229-234.
Magnetic modulators are mainly used together with other electronic circuits as sensitive, wide-band direct current amplifiers. The fields of application are mainly in industrial measurement and control installations, medical equipment and laboratory instruments where a high degree of accuracy and stability is demanded.
The action of a magnetic modulator is based on the known non-linear magnetization curve of magnetic materials. In its simplest form a magnetic modulator consists of a core of magnetic material with a primary and a secondary winding. The current which is, for example, to be measured is supplied to the primary winding, while a symmetrical reference alternating voltage signal such that the core is periodically driven symmetrically in both directions to magnetic saturation is applied to the secondary winding.
As a consequence of, for example, a direct current component in the current to be measured the core is, however, pre-magnetized in a direction which depends on the direction in which this direct current component passes through the primary winding. The magnetization current which is generated by the reference alternating voltage signal and which without premagnetization of the core has a symmetrical course will, on the other hand, be asymmetric in shape with a premagnetized core, as a consequence of the non-linear magnetization curve thereof. The degree of asymmetry is dependent on the degree of premagnetization. In other words, the magnetization current generated by the reference alternating voltage signal is modulated by the current in the primary winding. With the aid of further electronic circuits, the degree and the direction of the asymmetry in the magnetization current can be determined, by which means, for example, information on the strength and the direction of the said direct current component can be obtained.
In practice diverse circuits have been proposed for the detection of the asymmetry. In the article "Nullfluss-Stromwander zur Messung von Gleich- und Wechselstromen" ("Zero Flux Current Convertor for Measurement of Direct and Alternating Currents") by J. Lisser et al., Elektrotechnische Zeitschrift 100 (1979) no. 24, p. 1390-1394, a so-called second harmonic magnetic modulator is proposed in which use is made of the feature, known from Fourier analysis, that pure symmetrical alternating voltage signals can be considered to be made up solely of a series of odd harmonic components while a asymmetric signal will also contain even as well as odd harmonic components, the second harmonic of which is in general the strongest.
A pure sine-shaped reference alternating voltage signal, which, for example, can be derived from the mains frequency, is now applied to the secondary winding of the magnetic modulator, the voltage generated by the magnetization current over an impedance connected in series with the secondary winding being fed to a second harmonic band filter. The output signal from this filter, which is a measure of the strength of the second harmonic component in the magnetization current, is fed to a synchronous rectifier, to which the second harmonic of the sine-shaped reference alternating voltage signal is also fed. The magnitude and the polarity of the output signal from this synchronous rectifier are then a measure for the magnitude and the sign of the mean value of the magnetization of the core caused by the current in the primary winding.
A second harmonic magnetic modulator of this type has, however, a number of disadvantages. Without premagnetization, i.e. when the magnetization current is purely symmetrical, the output signal of the synchronous rectifier is equal to zero, as a result of which noise and other interfering signals have a relatively great influence on the further processing of the output signal thereof. Furthermore, very stable, sharply delimited band filters must be used because otherwise the third order harmonic, which can be relatively strong, is fed as an undesired signal to the synchronous rectifier. As is known, the construction of sharply delimited band filters, especially at relatively low frequencies in the order of magnitude of 50-500 Hz, is very expensive. Because synchronous rectification is also very phase-sensitive, electronic components with a very high accuracy and stability must be used, which makes the circuit extra expensive. The band width of the magnetic modulator is limited by the accuracy of the components used.
In order to prevent the reference signal inducing a voltage in the primary winding of the core, a second identical core is added, the primary windings of the two cores being connected in series and the modulating current passing through both. The reference alternating voltage signal is now fed to the secondary windings such that no resultant induction current is induced in the modulating current, i.e. that the induction currents generated by the two cores cancel one another out.
In another circuit for the detection of an asymmetry in the magnetization current generated by the reference signal, a peak value detector is used in place of second harmonic detection, as mentioned in the introduction. The action of this detector can be understood as follows.
With a magnetization curve with a relatively sharp saturation bend the magnetization current will suddenly increase in the saturation region of the core because the self-induction of the secondary winding falls rapidly beyond the saturation bend and the magnetization current is in fact then determined solely by the impedance connected in series with tee winding. When the magnetization current is symmetrical, the positive and negative periods of the magnetization current have the same shape. Peak value rectification of the individual half periods and summation of this rectified signal theoretically gives an output voltage of zero volts in the case of perfect symmetry. When, as a consequence of a premagnetization of the core, the magnetization current is no longer symmetrical in shape, the rectified signals of the individual half periods will, after summation, give an output signal which is not equal to zero. The polarity and the magnitude of this output signal correspond to the direction and the magnitude of the premagnetization.
The use of a peak value detector in the magnetic modulator has the advantage over the said second harmonic detection that the circuit as a whole can be appreciably less expensive because, in contrast to the said second harmonic detection circuit, in the peak value detector it is not harmonic signals but the magnetization current itself, generated with the reference voltage, which is processed. This means that no expensive band filters, synchronous rectifiers and auxiliary circuits have to be used to obtain a precisely sine-shaped reference voltage.
A disadvantage of the circuit with peak value detector is, however, that in practice the output signal from this detector, in the case of a purely symmetrical magnetization current, always gives an undesired ripple voltage with a frequency equal to the frequency of the reference voltage. The magnitude of this ripple voltage is determined by the desired response time of the circuit. For a rapid response, the peak value detector must possess a time constant as small as possible. This means that the electrical charge which is stored in the peak value detector and is proportional to the peak values of the magnetization current must be discharged relatively rapidly, which, however, results in an increase in the size of the outlet ripple.
As with the second harmonic magnetic modulator, a second identical core is used in a similar manner in the magnetic modulator with peak value detector in order to prevent influencing of the modulating current as a consequence of the reference voltage.